1. Field of the Invention
The present invention relates to a semiconductor GaN-based light emitting element containing gallium nitride and to a method of making the same.
2. Description of the Related Art
A semiconductor light-emitting element (a blue-color light-emitting diode) for blue-color light emission has been developed and put to practical use. It has a sapphire substrate on which a compound semiconductor crystal containing gallium nitride (GaN) is epitaxially grown by vapor-phase growth of an organic metal compound. Since sapphire differs only slightly in lattice constant from a GaN-based semiconductor crystal, it provides a suitable surface for epitaxially growing a GaN-based compound semiconductor layer while succeeding the crystal orientation of the sapphire.
However, since sapphire is expensive and has poor processability, a conventional diode for blue-color light emission fabricated with use of such a substrate is inevitably expensive.
On the other hand, JP-A-2000-31534 for example discloses a technique utilizing a silicon single crystal substrate as an inexpensive and readily processible substrate for making a blue-color light-emitting diode. According to this technique, a monocrystal silicon substrate is surface-treated with hydrogen and then formed with a titanium nitride layer as an intermediate layer for growth of a GaN-based semiconductor. Such hydrogen surface treatment makes hydrogen joined to dangling bonds on the surface of the silicon substrate, which prevents formation of titanium silicide which is an amorphous layer. Further, since titanium nitride has a cubic crystal structure similarly to silicon, titanium nitride can be appropriately grown on the silicon substrate while succeeding the crystal orientation of silicon. Further, by properly setting the thickness of the TiN layer, the GaN-based compound semiconductor layers subsequently formed thereon also succeed the crystal orientation of silicon.
In place of forming an intermediate layer of TiN as described above, the above document also proposes another method which utilizes an intermediate layer made of AlN/AlCaN for forming a blue-light-emitting diode by epitaxial growth of a GaN-based semiconductor on a monocrystal silicon substrate.
However, since silicon is a light-absorptive material as is generally known, the blue-color light-emitting diode disclosed in the above document still has room for improvement in light-emitting efficiency.
To improve light-emitting efficiency, JP-A-5-13816 for example proposes a technique which takes advantage of transparency of a sapphire substrate for emitting light from the sapphire substrate.
However, since sapphire is expensive and has a poor processability as described above, a blue-color light-emitting diode fabricated with use of a sapphire substrate is still expensive. Further, it is desirable to make the chip as small as possible in view of the high cost of the substrate while also increasing the current density passing through the activation layer. Then, some practical and useful way must be sought for properly mounting such a compact chip on a mother substrate or frame while maintaining the light-emitting efficiency.